Superconductive quantum interference device having two cavities isolated by a superconductive weak link

ABSTRACT

A symmetric superconductive quantum interference device (SQUID) for use in magnetic field, or radio frequency energy, sensing systems. The symmetric SQUID comprises a solid niobium cylinder with a dumbbell shaped hole formed longitudinally therethrough. A pair of niobium screws, one having a flat end and the other having a pointed end, are positioned in the cylinder normal to its longitudinal axis so as to contact and form a weak superconductive link in the narrow portion of the dumbbell shaped hole. The symmetric SQUID exhibits the quantum interference effect in response to a difference in magnetic field strength between its two holes (i.e. the two circular portions of the dumbbell shaped hole). Thus the device will respond directly to a magnetic field gradient, but not to a uniform magnetic field. Response to a uniform magnetic field can be achieved by inductively coupling a flux transformer to one hole of the device, so that in the presence of an applied field, flux shifts from one hole of the device to the other. Magnetic field gradients can be sensed by inductively coupling a gradientsensitive flux transformer to one hole of the SQUID. The gradient sensitive flux transformer may comprise a pair of superconductive loops each formed by a thin niobium sheet wrapped around a quartz tube. In another embodiment of the invention, suitable for making sensitive temperature measurements, cerous magnesium nitrate (CMN) powder is inserted into one hole of the symmetric SQUID. The SQUID is then cooled to its superconductive state in the presence of a magnetic field and then the field is removed. A change in temperature causes the susceptance of the CMN powder to change, thereby shifting magnetic flux lines from one hole of the device to the other. This shifting of flux causes a change in the impedance of the SQUID. This change can be measured to ascertain the temperature change.

United States Patent 11 1 Zimmerman 1 51 Sept. 11, 1973 [5SUPERCONDUCTIVE- QUANTUM INTERFERENCE DEVICE HAVING TWO CAVITIESISOLATED BY A SUPERCONDUCTIVE WEAK LINK [76] Inventor: James E.Zimmerman, 404 I-Iapgood Ave., Boulder, Colo. 80302 Mar. 12,1971 (UnderRule 47 211 Appl. No.2 123,686

[22] Filed:

[52] U.S. CI. 324/43 R, 128/206 E, 307/306,

OTHER PUBLICATIONS Zimmerman et al., Design'& Operation....Contacts,Journal of App. Phys, Vol.41, No. 4, pp. 1,572-1 ,580, Mar. 15, 1970Primary Examiner-Robert J. Corcoran AttorneyI-Ierbert Epstein [57]ABSTRACT A symmetric superconductive quantum interference device (SQUID)for use in magnetic field, or radio frequency energy, sensing systems.The symmetric SQUID comprises a solid niobium cylinder with a dumbbellshaped hole formed longitudinally therethrough. A pair of niobiumscrews, one having a flat end and the other having a pointed end, arepositioned in the cylinder normal to its longitudinal axis so as tocontact and form a weak superconductive link in the narrow portion ofthe dumbbell shaped hole. The symmetric'SQUID exhibits the quantuminterference effect in response to a difference in magnetic fieldstrength between its two holes (Le. the two circular'portions of thedumbbell shaped hole). Thus the device will respond directly to amagnetic field gradient, but not to a uniform magnetic field.

Response to a uniform magnetic field can be achieved by inductivelycoupling a flux transformer to one hole of the device, so that in thepresence of an applied field, flux shifts from one hole of the device tothe other. Magnetic field gradients can be sensed by inductivelycoupling a gradient-sensitive flux transformer to one hole of the SQUID.The gradient sensitive flux transformer may comprise a pair ofsuperconductive loops each formed by a thin niobium sheet wrapped arounda quartz tube.

In another embodiment of the invention, suitable for making sensitivetemperature measurements, cerous magnesium nitrate (CMN) powder isinserted into one hole of the symmetric SQUID. The SQUID is then cooledto its superconductive state in the presence of a magnetic field andthen the field is removed. A change in temperature causes thesusceptance of the CMN powder to change, thereby shifting magnetic fluxlines from one hole of the device to the other. This shifting of fluxcauses a change in the impedance of the SQUID. This change can bemeasured to ascertain the temperature change.

12 Claims, 20 Drawing Figures PATENTEDSEPI 1 ms sum 5 or 6 Ill llll

1 fit. nvriawu lllllilll IZICTKO [Aka/0619441 1 INVENTOR- c/A/l/[f 15'.Z/MMZRM/l/ SUPERCONDUCTIVE QUANTUM INTERFERENCE DEVICE HAVING TWOCAVITIES ISOLATED BY A, SUPERCONDUCTIVE WEAK LINK BACKGROUND OF THEINVENTION The invention herein described was made in the course of orunder a contract or subcontract thereunder, with the Advanced ResearchProjects Agency, Washington, D. C.

The invention relates to an improved superconductive quantuminterference device (SQUID).

The SQUID is a low-temperature circuit element capable of exhibitingperiodic variations in its electrical characteristics as a function ofapplied magnetic field or drive current. These devices can be used fordetection of magnetic fields or generation of electromagnetic fields, oras a non-linear circuit element for detection or mixing of R.F. signals.

SQUIDS comprise superconductive structures separated by a weaksuperconductive link. The weak link may be formed by positioning a verythin insulating layer between two superconductive structures or bypoint-contacting one of the superconductive structures to a flat surfaceof another of the superconductive structures. Point contacts formed byusing a pair of superconductive screws, one having a pointed end and theother having a flat end are especially'useful because the pressure ofthe contact, on which electrical properties of the device depend, can beeasily adjusted.

Prior-art point contact devices have proved to be unreliable andunstable because of the sensitivity of the electrical characteristics ofthe device (particularly the critical current) to the mechanicalcharacteristics of the point contact. (By critical current is meant thatcurrent at and above which the device behaves as a normal conductor ascontrasted to a superconductor); The mechanical characteristics areadversely affected by atmospheric oxidation or corrosion of the contact,deformation of the contact by differential thermal expansion orcontraction of the supporting structure due to recycling of the devicebetween room temperature and the cryogenic temperature at whichsuperconductivity is achieved (typically near 4K), and deformation ofthe contact by mechanical vibration of the supporting structurep Anotherobject is to provide a SQUID and flux transformer combination withincreased sensitivity to magnetic field gradients.

Another object is to provide a SQUID system capable of sensitivetemperature measurement.

" Another object is to provide an improved method for measurement oftemperature.

DRAWINGS OF THE INVENTION FIG. 1 is an end view of a symmetrical SQUIDaccording to the invention.

FIGS. 2 and 3 are sectional views of said symmetrical SQUID.

FIG. 4 is a picture of a measured radio-frequency (R.F.) voltage-RFcurrent (V-I) characteristic of said symmetrical SQUID.

FIG. 5 is a picture of R.F. voltage D.C. magnetic field characteristicsof said symmetrical SQUID.

FIG. 6 is a schematic diagram of a point contact superconductive ringcoupled to a resonant circuit.

FIG. 7 is a graph of the ratio of the total static flux enclosed in apoint-contact superconductive ring to a quantum of magnetic flux as afunction of the ratio of the external magnetic flux enclosed by the ringto a quantum of magnetic flux.

FIG. 8 depicts the theoretically-predicted R.F. voltage of a pointcontact superconductive ring plotted as a function of R.F. currentthrough the contact and of static magnetic field through the ring.

FIG. 9 is a block diagram of the apparatus used to measure theelectrical characteristics of a SQUID.

FIG. 10 is a block diagram of a magnetometer which incorporates asymmetric SQUID according to the invention.

FIG. 11 is a schematic diagram of a flux transformer used in combinationwith a symmetric SQUID according to the invention to measure magneticfield strength.

FIG. 12 is a schematic diagram of a flux transformer used in combinationwith said symmetric SQUID to measure magnetic field gradient.

FIG. 13 is a spatial schematic diagram of that gradient-sensitive fluxtransformer.

devices is due to the effect of anomolies in the'ambient magnetic field,variations in the ambient magnetic field due to electronic circuitinstabilities (e.g. drift'in line voltage or component parameters), andnoise introduced by the proximity of conducting bodies to the measuringdevice. v

Accordingly an object of this invention is to provide an improved SQUID.

Another object is to provide a SQUID which is insensitive to thermalcycling.

Another object is to provide a SQUID which is insensitive to mechanicalshock. Another object is to provide a SQUID with increased sensitivityto magnetic fields. I

Another object is to provide a SQUID and flux transformer combinationwith increased sensitivity to magnetic fields.

FIG. 14 is a more structural diagram of that transformer, and FIG. 14Adepicts superconductive loops V forming part of that transformer.

FIG. 15 is a block diagram of an apparatus comprising the symmetricSQUID, usedto make field measurements of a magnetic field gradient.

FIG. 16 depicts the phase-shift versus inverted tem-- paratus using thesymmetric SQUID of the invention.

FIG. 19 depicts an electrocardiagram taken from the person from whom themagnetocardiogram of FIG. 19 wasv taken. I

DESCRIPTION OF THE SYMM'ETRIC SQUID The symmetric SQUID of the inventionis shown in FIGS. 1, 2, and 3.

The SQUID comprises cylindrical body 20, which is composed of thesuperconductive material niobium. Body 20, otherwise tubular, contains asolid central portion 21 within which is longitudinally disposed adumbbell shaped hole 22 comprising circular portions 24 and 34 andnarrow connecting portion 32. A weak superconductive link is formed byniobium screws 26 and 36, which are coaxially mounted in threaded holesdrilled and tapped transversely across the center of portion 21 andwhich contact in opposing relationship the narrow connecting portion 32of dumbbell shaped hole 22. Screw 26 has a pointed end which may beformed on a jewelers lathe and honed to microscopic sharpness. Screw 36has a fiat end which is contacted by the pointed end of screw 26 to formthe weak link. When the screws are adjusted to yield desired contactpressure, they may be locked into place by niobium nuts 28 and 38 whichfit into slots 30 and 40 formed in cylindrical body 20. Holes 42 and 44,which are formed in cylindrical body 20, allow magnetic shielding of thepoint contact formed in dumbbell shaped hole 22 by providingsuperconductive structure (i.e. the walls of cylindrical body 20)extending beyond the depth of the dumbbell shaped hole. Where suchshielding is not desired, e.g. in certain gradiometers, the walls ofcylindrical body 20 may be'coextensive with the sides of portion 21 and,i.e.' dimension equal to and coincident with dimension b.

Examplary device dimensions and screw sizes are listed in the followingtabulation (Table I). However devices having different dimensions andscrew sizes can also be used.

TABLE I Device Dimension Size a. 0.080" b /16" III d %I! e *n Screw 26OOO-IZO 36 GOO-I20 I The device of the invention is inherently highlyshock resistant because of its shape. The two opposing mem bers of thepoint contact, screws 26 and 36, are held in precise relativeposition'by the rigid symmetrical double bridge structure formed by bodyportion 21 and dumbbell shaped hole 22. The screws themselves areisolated from directmechanical shock by being recessed within slots 30and 40.

Since the electrical characteristics of the SQUID are a sensitivefunction of point contact pressure, the mechanical rigidity of thedevice of the invention results in stable electrical operation.Experimentally it has been determined that transverse compressive forcesin excess of l kilogram, applied to the sides of body 20, result in nomeasureable effect on the contact.

Another cause of instability in point contact SQUIDS is thermaldeformation of the point-contact due to cycling between room temperatureand the cryogenic temperature (about 4.2K for niobium) at whichsuperconductivity takes place. The device of the invention is inherentlyresistant to thermal deformation because all of the structures whichform and support the point contact (viz., body 20, screws 26 and 36, andnuts 28 and 38) are composed of the same superconductive material, viz.,niobium, and therefore have the same thermal expansion characteristics.This fact has been proven experimentally by repeated temperature cyclingof such devices. One device has been cycled between RESPONSE OFSYMMETRICAL SQUID TO APPLIED MAGNETIC FIELDS The usefulness of a SQUIDas a circuit element is based on its capability of exhibiting theso-called quantum interference effect i.e. when cooled to thetemperature at which it becomes a superconductor, the device exhibitsperiodic variations in its electrical characteristics as a function ofapplied magnetic field or drive current. However, due to its particularshape, the symmetric SQUID of the invention differs from prior-artSQUIDS in its response to applied magnetic fields in that it does notrespond to magnetic fields, whether constant or time-varying, whosevalue at any instant is the same in both holes thereof. Instead, thesymmetric SQUID exhibits the quantum interference effect in response toa difference in magnetic field strength between its two holes. Thisdifference may be brought about by applying a magnetic field gradient(i.e. a field which varies spatially) directly to the device or bycausing flux to shift from one hole to the other (e.g. by inserting aflux sensing coil in one of the holes).

The symmetric SQUID has these unique properties because it isfunctionally equivalent to two superconducting rings (formed by theniobium bounding holes 24 and 34) connected in parallel with a commonweak link (formed in narrow connecting portion 32 by screws 26 and 36).When the symmetric SQUID is above its superconductive temperature, itsniobium body acts as a normal non-magnetic metal and passes magneticflux lines without distortion of those lines. However, when the deviceis cooled below its superconductive temperature in the presence of auniform magnetic field directed through the holes the number offlux-lines trapped in holes 24 and 34 by the cooling will be equal(because the device has a symmetrical structure) and hence no differencein field strength will exist across the weak link. A difference in fieldstrength can be applied across the weak link by inserting a coil intoone of the holes of the device and supplying R.F. energy to that coil.The electromagnetic field surrounding the coil will increase the flux inthat hole and create a flux difference across the weak link. Analternate way of creating the flux difference is by use of a fluxtransformer which comprises a superconductive wire shaped at one endinto a loop for sensing an external magnetic field and shaped at theother end into a coil which is inserted into one of the holes of theSQUID for supplying flux from that field to that hole. In either case,when the resulting differential field reaches a value defined as thecritical value the weak link behaves as a normal conductor, exhibitingappreciable electrical impedance between screws 26 and 36. When the linkbecomes normally conducting, the flux redistributes itself across thelink so as to eliminate the flux difference. Once the flux difference iseliminated, the weak link again returns to its superconductive state(wherein it again exhibits no appreciable electrical impedance betweenscrews 26 and 36) until a further flux application again creates a tivering when cooled in a uniform magnetic field. The

behavior ofa superconductive ring in a magnetic field can be explainedby the theory of Meissner currents.

This theory postulates that, as a body of superconductive material ofany shape is cooled from above the superconductive temperature to belowthat temperature, while the body is within a uniform magnetic field,circulating electric currents will appear on the surfaces of that body.

These surface orMeissner currents are of such magnitude and are sopositioned as to create an opposing magnetic field that exactly'balancesthe applied field and in effect cancels or expels it from thesuperconductive material. In the case of a tubular body having an innersurface and an outer surface, e.g. a ring, two Meissner currents appear,one on the inner surface, the other on the outer surface. The Meissnercurrent on the inner surface of the ring effectively pushes" the fluxinto the rings interior while the Meissnercurrent on the outervsurfaceof the ring pushes the flux away from the outer surface. When theapplied magnetic field is removed from the ring (the temperature of thering being below the superconduction temperature) the outer surfacecurrent disappears but the inner surface current remains, therebytrapping the flux inside the ring untilthe superconductive material isdriven into normal conduction by application of a large external fieldor is allowedto become normally conductive by permitting it to warm to atemperature above the superconduction temperature.

In contrast, in the case of the symmetric SQUID cooled in the presenceof a uniform field, there is no Meissner current in the weak linkbecause the Meissner currents in the two internal rings (holes 24 and34) are equal and in the same direction (e.g. both clockwise), andtherefore cancel each other in the weak link. When the field differenceacross the weak link is introduced, the Meissner currentin the weak linkincreases until-it reaches the critical current value for which the'linkgoes out of superconduction and into normal conduction, i.e. the normalstate. At that point the flux redistributes itself, thereby decreasingthe Meissner currentbelow the critical value so that the link againreturns to its superconducting state.

Since the two superconducting rings formed by holes 24 and 34 aresurrounded by the relatively massive superconducting ring formed bycylindrical body 20, or-

dinarily encountered fields which are I sufiiciently strong to drive theweak link into normal conduction when sensed by a flux transformer willnot be sufficiently strong to drive the entire structure into the normalstate. Thus the symmetric SQUID will be effectively shielded fromexternal uniform fields and thereby the sensitivity of field measuringdevices using the symmetric SQUID is greatly enhanced.

While the advantageous properties of the symmetrical SQUID are achievedin the preferred embodiment of the invention by a device comprising acylindrical body and a dumbbell shaped hole it will be apparent to thoseskilled in the art that other body and hole shapes can be used so longas symmetry about the weak link is maintained.

ELECTRICAL CHARACTERISTICS In an external magnetic field the symmetricSQUID exhibits the electrical characteristics of a superconductive ringhaving a weak superconductive link. The latter device is useful as amagnetic flux detector because critical current of the device, whenwithin a magnetic field, varies periodically, the period being'dependent on the amount of flux threading the superconductive ring. Byelectrically biasing the device within the range of critical currentswhich result from the range of expected variations in magnetic field,the device will be caused to go through successive periods of normalityand superconductivity as a function of external field. The bias currentcan be supplied by making direct contact with the SQUID or byinductively coupling a radiofrequency (R.F.) bias signal to the SQUID byinserting a coil driven by an R.F. generator into one of the holes. Asthe external field is varied, the excursions between superconductivityand normality will cause the device to exhibit a periodic variation inimpedance and voltage drop between screws 26 and 36. By sensing thisvoltage or by using it to generate another magnetic field to cancel outthe external field in the ring, a measure of the external field can beobtained.

The mechanism of RF. biased superconductive rings which contain a weaklink, their characteristics, and methods for using such devices formeasurement of magnetic fields are discussed fully in the publication,Quantum States and Transitions in Weakly Connected SuperconductingRings, by A. H. Silver and J. E. Zimmerman, The Physical Review, Vol.157, No.2, pages 317-341, May 10, 1967. Hence no further discussionthereof is necessary herein.

FIGS. 4 and 5 are pictures of the electrical characteristics of thesymmetrical SQUID of the invention as displayed by a cathode-rayoscilloscope. FIG. 4 shows the V-] characteristic of the device, i.e.detected R.F. voltage across the device versus the amplitude of RF drive(or bias) current. The bias current was applied by the aforementionedmethod of inductive coupling, using a 30 MHz bias signal. The variationin amplitude of the bias signal, depicted along the abscissa of FIG. 4,was obtained by amplitude modulating the RF. drive signal with an audiofrequency signal. The V-I curve oscillates between two extremes as afunction of DC. applied field. The oscillation is shown directly in FIG.5, in which the four respective curves are plots of the detected R.F.voltage as a function of DC. field strength, for four different fixedamplitudes of RF. drive current. In this measurement theD.C. field isvaried by driving the inductive coupling coil with an audio signal. Eachtrace in FIG. 5 corresponds to a loop in FIG. 4 at the same detectedR.F. voltage level. The phase reversal between successive traces in FIG.5 corresponds directly with the successive cross-overs of the two tracesof FIG. 4.

The reasons for the electrical characteristics shown in FIGS. 4 and 5will be understood better from the following analysis of the operationof the circuit shown schematically in FIG. 6. The circuit of FIG. 6comprises a superconductive ring 46, having a point contact 48.

The self inductance of the superconducting ring is designated L. Ring 46is inductively coupled to the inductor 54 of a resonant (or tank)circuit 50. The mutual inductance of this coupling is designated M.Resonant circuit 50 comprises the parallel combination of inductor 54,having inductance L,, a capacitor 56 having capacitance C, and a loadresistor 58 having resistance R The behavior of the superconductive ringin a magnetic field is similar to the behavior of the symmetric SQUID ofthe invention, except that the ring responds directly (i.e. without needof a flux transformer) to a uniform magnetic field. Tank circuit 50represents the equivalent circuit of all external measuring circuitrywhich is inductively coupled to the ring. Inductor 54 represents theself inductance. of a coil which is inserted in the ring (or SQUID), andcapacitor 56 and resistor 58 respectively represent the totalcapacitance and resistance connected across the coil. Circuit 50 isdriven by a source 52 of R.F. current having a frequency w 0.

In the aforementioned paper by Silver and Zimmerman, the authors showedthat the dynamic behavior of the device can be derived from aquantum-periodic magnetic response function shown in FIG. 7, which iseither continuous and reversible or discontinuous and hysteretic,depending upon whether thecritical current, i of the point contact isless than or greater than ,/2L(l 'y). In FIG. 7, the ratiolo is plottedas a function of the ratio (b /(b0, where d) is the total static flux,(1), is the externally applied flux, and 420 represents the smallestunit of magnetic flux, 1 quantum, numerically equal to h/2e, where h isPlancks constant and e is the charge on anelectron. y is a dimensionlessparameter dependent on the inductance of the ring and the size of theweak link. In practical devices 7 is small compared to unity. For islightly greater than d ,/2L(l 7) transitions between the disconnectedstates shown in FIG. 7 obey a selection rule A n i 1; that is,transitions take place only between adjacent states. Where i, is greaterthan b, ./2L (I 'y) and where the R.F. bias frequency (n is set equal tothe low-level resonant frequency of system, the device is equivalent toa shorted turn in the field of the coil 54. Therefore (n is higher thanthe resonant frequency of the tank circuit in the absence of the device,or with the contact open, or at high bias levels. The R.F. voltage V,across the tank circuit is a linear function of the R.F. bias current iI, into the tank circuit as long as the total peak current in thesuperconducting loop is less than i For the case I where the externalapplied field is D.C., and:

(if; da (or nd the critical current I, is reached at the flux level(taking y -0):

The R.F. flux 41, applied to the device via the R.F. bias current is:

1= Mi, V, M/w L,

where i, is the current in tank coil 54. Hence n M/w. L, Li,

Denoting this value of V, as V and the corresponding value of I, as I,,,we have:

ic 7 e l a/ n: RL

The expected R.F. voltage amplitude V, as a function of R.F. current I,and of external static field d) D.C. is shown in FIG. 8. At the biaslevel of equation 1, viz., Vi li (FIG. 8, point A) a transition to oneof the two adjacent states and back takes place. Thus, the tank circuitenergy is abruptly reduced by the area of one hysteresis loop, that isThis is equivalent to shock-exciting the tank out-ofphase with thedriven oscillation. No further transitions can take place until theshock excitation dies out, that is, until the oscillation level againbuilds up to the critical level. Consequently, the system undergoes alowfrequency, low-ambient saw-tooth modulation in time. As the biascurrent I, is further increased above the critical value, the buildup ofthe oscillation level V, is more rapid and the saw-tooth modulationfrequency increases. However, the modulation amplitude and the averageoscillation level V, remain fixed, the former being proportional to A Eand the latter being essentially equal to V minus half the modulationamplitude. Both A E and V,, are fixed parameters of the system. Thus V,is limited at this level until two hysteresis loops, one above and onebelow the D.C. field level da are traversed on every R.F. cycle. At thispoint (FIG. 8, point B), V, again increases until the second pair ofhysteresis loops is encountered, at which point (FIG. 8, point C) V, isagain limited by the mechanism described above. The total response maybe described as a linear rise in V, interrupted by an equally-spacedseries of plateaus.

Similarly, if da /d) 0 is an odd half-integer, dc )0r then the firstplateau comes near:

In this case, limiting is effected by one hysteresis loop ratherthanitwo, so the first plateau (FIG. 8, points D to E) is half as longas succeeding ones.

Finally, it can be shown by extension of these arguments that for aparticular R.F. bias level (FIG. 8, point F, for example), V, increaseslinearly in the regions (n k) (n l) ancl decreases linearly in theregions (n d l n. That is, the response as a function of D.C. field is atriagnular wave. Furthermore, the wave reverses phase as the R.F. levelis increased so as to encompass the next adjacent pair of hysteresisloops, or so that the R.F. flux amplitude at the superconducting loopincreases by ,,/2 (FIG. 8, point G).

This theory is valid only if the tank circuit has a fairly high Q, andthe mutual inductance M is large enough that the inherent energy lossper cycle, 2 1r V, 2/0, R is considerably smaller than the area of ahysteresis loop.

The experimental results shown in FIGS. 4 and 5 are typical of patternsobtained with symmetrical SQUIDS of the type shown in FIGS. 1-3, with atank circuit Q of about 100, M/ V LL, =0.05, L=4 l0" h, an overallsystem bandwidth of 1.0 Hz, a tank circuit capacitance C 2 X l0-farad,and (0,, 30MHz. The tank coil, a lO-turn coil of No. 38 copper (about lmm CD.) was inserted directly in one hole of the device.

The experimental results shown in FIGS. 4 and were obtained with theapparatus shown in FIG. 9. In this apparatus the cryogenic portion,which is cooled to 4.2I(, is shown within outline 78. This portion ofthe apparatus is connected to the external measuring circuitry, at roomtemperature, by coaxial cable 86. The tank circuit inductance L issupplied by coil 82 which is inserted directly into one hole,represented schematically by dotted outline 80a, of symmetric SQUID 88.Tank circuit capacitor 84 is connected in parallel with coil 82. Theparallel combination of coil 82 and capacitor 84-is connected by coaxialcable 86 to the input of preamplifier 68 where the signal existing inthe tank circuit is amplified by low noise amplifiers. The tank circuitwhich comprises coil 82, capacitor 84 and the input impedance topreamplifier 68, is tuned to 30 MHz. Preamplifier 68 is connected toamplifier 70 where the 30 MHz signal is further amplified. Amplifier 70is connected to detector and filter circuit 72 where any amplitudemodulation present in the 30 MHz signal is detected. The output ofcircuit 72 is representative of the amplitude of the 30 MHz carriersignal. Circuit 72 is connected to the Y (or vertical) plates of XY-scope 78 where the amplitude of the 30 MHz signal existing across thetank circuit is displayed. Oscillator 62, which supplies the 30 MHzsignal used for biasing the SQUID, is connected to coaxial cable 86 byloose capacitive coupling 66. The loose coupling provides a highimpedance connection to the tank circuit which assures that the biascurrent is supplied from a constant current source (ie so that the biaslevel will not be a function of tank circuit impedance). Audiooscillator 74 is connected by switch 76 to either oscillator 62(position a) or choke 64 (position b). When the V-I characteristic (FIG.4) is to be measured, switch 76 is thrown to position a, and the audiosignal is used to amplitude modulate the output of oscillator 62. This,in effect, varies the bias current supplied to the SQUID. Audiooscillator 74 is also connected across the X (or horizontal) axis of XYscope 78. Thus when switch 76 is connected in position a, the variationin the amplitude of the SQUID bias current is displayed on the X- axison theface of scope 78 and the variation in voltage across the tankcircuit is displayed across the Y-axis (as shown in FIG. 4). When thefield characteristic (FIG. 5) is to be measured, switch 76 is thrown toposition b, and the audio signal, which is applied via choke 64 andcoaxial cable 86 to coil 82, is used to vary the field strength in theSQUID. In this manner, the variation in field strength is displayed onthe X-axis on the face of scope 78 and the variation in voltage acrossthe tank circuit is displayed across the Y-axis (as shown in FIG. 5).

USB or THE SYMMETRIC SQUID AS A MAGNETOMETER AND GRADIOMETER Byreplacing the X-Y scope, in the circuit of FIG. 9, by a phase-lockeddetector and feeding the DC. output of the phase-locked detector back,to the SQUID, the system can be made to function as a lock-onmagnetometerfThe magnetometer circuit is shown in FIG. 10. Componentssimilar to those of the FIG. 9 circuit are similarly numbered.

18 A suitable phase locked detector 88' is a Princeton Applied ResearchModel HR8, having the functional The phase locked detector output iscoupled back to the tank circuit coil 82 of the SQUID by couplingresistors 90 and 92, choke 64, and coaxial cable 86. This output is alsoapplied to chart recorder 98 by the low pass filtering networkcomprising resistor 94 and capacitor 96.

In operation, changes in external field which are sensed by theflux-transformer will cause the phaselocked detector to produce a DCoutput signal which is fed back to coil 82 to produce a field in theSQUID which tends to oppose the external field until the output of thephase detector isnulled. The change in field strength needed to opposethe external field is recorded on chart recorder 98 as the magnetometeroutput.

The external field is applied to the symmetric SQUID 80a by inserting inthe other hole of the SQUID a coil 100 which is coupled to fluxtransformer 102. The flux transformer-SQUID interconnection isrepresented schematically in FIG. 11. The flux transformer is a closedloop comprising a superconductive loop, repre' sented schematically bycoil 104, connected to coil I06. Coil 186 is inserted into one of theholes of symmetrical squid 1108 thereby magnetically coupling thefluxtransformer in the SQUID by mutual inductance M. Agradient-sensitive flux transformer is shown schematically in FIG. 12.The external flux sensing portion of the gradient sensitive fluxtransformer is represented by two coils I10 and 112, of equalinductance, connected in series opposition. Thus, there is no responseto a uniform applied field, but a current proportional to the gradientof the field will be induced. The gradient-sensitive flux transformercan be represented spatially as shown in FIG. 13, where the two-externalsuperconductive loops, 114 and 116, are aligned coaxially. The fluxtransformer responds to 8 Bz/8 Z, where z represents distance along theline throughthe center of the two loops, and Bz represents the fluxdensity of the external field as a function of z. The sensitivity ofthis device varies inversely with the distance between the sensingloops, 1.

The gradient-sensitive flux transformer used in the preferred embodimentof the invention is shown in FIGS. 14 and 14A. The flux sensingsuperconductive loops, and 124, are each formed by a single turn of thinniobium sheet wrapped on quartz tube 118. Connection to the SQUID ismade by twisted niobium leads 122. These leads are connected to a fluxsensing coil (not shown) which is inserted into one hole of thesymmetric SQUID. The flux sensing coil comprises five turns of No. 36'(0.25 mm) niobium wire. The dimensions of the flux transformer of FIG.14 are shown in Table 3.

TABLE 3 DEVICE DIMENSION SIZE millimeters f 50 l which contained liquidhelium 140. Dewar 138 was mounted inv glass dewar 134 which containedliquid nitrogen 136. The entire cryogenic apparatus was mounted incylindrical aluminum shield 132, to prevent interference from radio andradar apparatus. The shield must be placed symmetrically about thecryogenic apparatus to yield optimum results. The mounting apparatus(not shown) comprised non-metallic materials such as micarta sheets andnylon screws. Battery package 126 supplied power to electronic measuringcircuitry 128. This circuitry was the same as that shown in FIG. 9,except for the addition of a gain-of-IO- differential post detectionamplifier (following detector and filter circuit 72 of FIG. 9). Chartrecorder 130 was connected in parallel with the oscilloscope containedin circuitry 128.

The field measurements indicate sensitivities of the order of 10gauss/cm. Sensitivities of better than lgauss/cm. have beentheoretically predicted for this device.

OTHER APPLICATIONS OF THE SYMMETRIC SQUID The symmetric SQUID can beused for sensitive measurement of temperature by inserting into one holeof the device a magnetic material whose susceptance varies withtemperature. The device is then cooled to its superconducting state inthe presence of a magnetic field. Once in the superconducting state, theflux lines will be trapped in the device and the external field can beremoved. Then, aschanges in temperature cause changes of susceptance ofthe magnetic material, flux lines will shift across the point contact ofthe device. The changes in the interference pattern of the device as afunction of temperature, which can be measured by the circuitryheretofore described, will yield a sensitive measure of temperature.

In one embodiment of the invention, 6.6 milligrams of cerous magnesiumnitrate (CMN) was inserted into one hole of the SQUID. A magnetic fieldof 300 gauss was applied to the SQUID by means of a coil wound aroundits outer cylindrical surface. The device was then cooled to 4.2K. andthen the applied field was removed. Interference pattern measurements,as a function of temperature, were made using the circuitry of FIG. 9.The results are shown in FIG. 16. The deviations from the striaght lineof FIG. 16 are accounted for by imprecision in measurements of phaseshift and temperature. The inherent resolution of the symmetric SQUID asa temperature sensor has been computed to be of the order of degrees. lna similar manner the device can be used to accurately measuresusceptance of magnetic materials.

It will be apparent to those skilled in the art that the symmetric SQUIDcan be used as an ultrasensitive magnetometer in other applications,such as the examination of airlines passengers for metal objects or themeasurement of the magnetic field associated with the human heart. Forexample, the device was used in a probe shown in FIG. 17 as part of amagnetocardiograph. The probe was connected to external electroniccircuitry (not shown), similar to that shown in FIG. 9, by stainlesssteel coaxial cable 148. A lO-turn coil of copper wire 154 waspositioned in one hole of SQUID 152 and coupled to coaxial cable 148 andtank circuit capacitor 150. Flux transformer 156, which was constructedof 0.25 millimeter niobium wire, was coupled to the. other hole of theSQUID. The entire lower end of the probe was surrounded by mylar sleeve158. The

probe was positioned in front of the subject's chest. I

FIG. 18 shows the magnetocardiogram obtained from a human subject. Itcan be seen that the magnetocardiogram pattern compares favorably withthe electrocardiogram, of the same subject, shown in FIG. 19. Oneadvantage of the magnetocardiograph is that terminals need not beconnected to the subject.

The symmetric SQUID can also be used as a detector of R.F. energy. Inthis application the device can be used to terminate a section ofwaveguide. When R.F. is applied to the waveguide its presence isdetected as a change in impedance of the tank circuit used with thedevice. Alternatively, the R.F. signal to be detected can be applied toone end of the device while a local oscillator signal of slightlydifferent frequency is applied to the other end of the device. The tankcircuit can then be tuned to the difference between these twofrequencies, to provide an I.F. pick off.

I claim:

1. A superconductive quantum interference device comprising:

a body of superconductive material having a plurality of holes extendingtherethrough, said body further having wall structure defining anaperture extending through said body, said aperture connecting at leasta pair of said holes, and a superconductive weak link connected acrosssaid aperture.

2. A superconductive device comprising:

a rod of superconductive material having a hole extending longitudinallytherethrough, the cross sec tion of said hole having a dumbbell shape inthe plane perpendicular to the longitudinal axis of said rod and beingdisposed symmetrically about said longitudinal axis; and

,first means for establishing a superconductive weak link across saidhole.

3. The device of claim 2 wherein said first means is composed of thesame superconductive material as said rod.

4. The device of claim 3 wherein said first means comprises means forestablishing a point contact across the center of said hole.

5. The device of claim 3 wherein said first means comprises first andsecond screws threaded trans versely through the center of said rod andmeeting in the narrow portion of said dumbbell shaped hole.

'6. The device of claim 5 wherein the end of said first screw whichextends into said rod'is pointed, and the end of said second screw whichextends into said rod is flat.

7. The device of claim 6 wherein said first means additionally comprisessecond and third means for locking said first and second screwsrespectively into position.

susceptance which varies with temperature.

11. The device of claim 10 additionally comprising a coil wound aroundthe outer cylindrical surface of said rod.

12. The device of claim 10 wherein said magnetic material is cerousmagnesium nitrate powder.

t l 8 l TIFIQATET OFyCQRRECTION ;;;.;*s358354 md V September 11, '1973OFFICE is? certified that; rrorjppeafs it the alaove identified patent"and thgt said Letiz ter'asP atent are hereby 'corr ec ted as shownbelow? v lint $0, "sehsinf' to producing f chng "imp e iance" toresistance I changi g "{D 2" 13:; (6 /2 505 change "tiriagnular" toigangular 0016155110, fil ihe. 36, after "14'." .1 start new v paragraphsighed and saled, this 1st day of ,October 1974.

(315%)) Attestigfl v MCQOY M; GIBSON JR, 1 c. MARSHALL DANN Atte's tiiigOfficer Commissioner of Patents 3 M mTEDfsTAT Sinew omen [[GEhH L AT 0UREQHQN met-m; "5,758,854 7 Dated September 11, 1973 Inventofl's) JamesE. Zimmerman It is certifies that error appears in the aboveidentifiedpatent and that said Letters Patent" are hereby corrected as shownbelow:

Q Column 14, line 30, change "sensing" to producing Colnmnfi, line 66,chenge "impedance" to resistance h Colnmn line 39, chenge "(6, 2" to 93/2 ColumnB, line 50, change "triagnulat" to ctiangular Column 10, line36, after "M'.' start new paregrap'h.

Signed and sealed this 1st day of October 1974.

(SEAL) Attest; V

MCCOY Mo GIBSON JR, 7 7C. MARSHALL DANN Attesting Officer I Commissionerof Patents

1. A superconductive quantum interference device comprising: a body ofsuperconductive material having a plurality of holes extendingtherethrough, said body further having wall structure defining anaperture extending through said body, said aperture connecting at leasta pair of said holes, and a superconductive weak link connected acrosssaid aperture.
 2. A superconductive device comprising: a rod ofsuperconductive material having a hole extending longitudinallytherethrough, the cross section of said hole having a dumbbell shape inthe plane perpendicular to the longitudinal axis of said rod and beingdisposed symmetrically about said longitudinal axis; and first means forestablishing a superconductive weak link across said hole.
 3. The deviceof claim 2 wherein said first means is composed of the samesuperconductive material as said rod.
 4. The device of claim 3 whereinsaid first means comprises means for establishing a point contact acrossthe center of said hole.
 5. The device of claim 3 wherein said firstmeans comprises first and second screws threaded transversely throughthe center of said rod and meeting in the narrow portion of saiddumbbell shaped hole.
 6. The device of claim 5 wherein the end of saidfirst screw which extends into said rod is pointed, and the end of saidsecond screw which extends into said rod is flat.
 7. The device of claim6 wherein said first means additionally comprises second and third meansfor locking said first and second screws respectively into position. 8.The device of claim 7 wherein said second and third means are nuts. 9.The device of claim 8 wherein said superconductive material is selectedfrom the group consisting of niobium, vanadium, and tantalum.
 10. Thedevice of claim 2 additionally comprising magnetiC material in one ofthe circular portions of said dumbbell shaped hole; said magneticmaterial having a susceptance which varies with temperature.
 11. Thedevice of claim 10 additionally comprising a coil wound around the outercylindrical surface of said rod.
 12. The device of claim 10 wherein saidmagnetic material is cerous magnesium nitrate powder.
 14. The other holeof the SQUID was magnetically coupled to external electronic measuringcircuitry 128 by coaxial cable
 142. The SQUID-flux transformercombination was mounted in glass dewar 138 of about 2 liter capacity,which contained liquid helium
 140. Dewar 138 was mounted in glass dewar134 which contained liquid nitrogen
 136. The entire cryogenic apparatuswas mounted in cylindrical aluminum shield 132, to prevent interferencefrom radio and radar apparatus. The shield must be placed symmetricallyabout the cryogenic apparatus to yield optimum results. The mountingapparatus (not shown) comprised non-metallic materials such as micartasheets and nylon screws. Battery package 126 supplied power toelectronic measuring circuitry
 128. This circuitry was the same as thatshown in FIG. 9, except for the addition of a gain-of-10-differentialpost detection amplifier (following detector and filter circuit 72 ofFIG. 9). Chart recorder 130 was connected in parallel with theoscilloscope contained in circuitry
 128. The field measurements indicatesensitivities of the order of 10 10 gauss/cm. Sensitivities of betterthan 10 11 gauss/cm. have been theoretically predicted for this device.OTHER APPLICATIONS OF THE SYMMETRIC SQUID The symmetric SQUID can beused for sensitive measurement of temperature by inserting into one holeof the device a magnetic material whose susceptance varies withtemperature. The device is then cooled to its superconducting state inthe presence of a magnetic field. Once in the superconducting state, theflux lines will be trapped in the device and the external field can beremoved. Then, as changes in temperature cause changes of susceptance ofthe magnetic material, flux lines will shift across the point contact ofthe device. The changes in the interference pattern of the device as afunction of temperature, which can be measured by the circuitryheretofore described, will yield a sensitive measure of temperature. Inone embodiment of the invention, 6.6 milligrams of cerous magnesiumnitrate (CMN) was inserted into one hole of the SQUID. A magnetic fieldof 300 gauss was applied to the SQUID by means of a coil wound aroundits outer cylindrical surface. The device was then cooled to 4.2*K. andthen the applied field was removed. Interference pattern measurements,as a function of temperature, were made using the circuitry of FIG. 9.The results are shown in FIG.
 16. The deviations from the striaght lineoF FIG. 16 are accounted for by imprecision in measurements of phaseshift and temperature. The inherent resolution of the symmetric SQUID asa temperature sensor has been computed to be of the order of 10 5degrees. In a similar manner the device can be used to accuratelymeasure susceptance of magnetic materials. It will be apparent to thoseskilled in the art that the symmetric SQUID can be used as anultrasensitive magnetometer in other applications, such as theexamination of airlines passengers for metal objects or the measurementof the magnetic field associated with the human heart. For example, thedevice was used in a probe shown in FIG. 17 as part of amagnetocardiograph. The probe was connected to external electroniccircuitry (not shown), similar to that shown in FIG. 9, by stainlesssteel coaxial cable
 148. A 10-turn coil of copper wire 154 waspositioned in one hole of SQUID 152 and coupled to coaxial cable 148 andtank circuit capacitor
 150. Flux transformer 156, which was constructedof 0.25 millimeter niobium wire, was coupled to the other hole of theSQUID. The entire lower end of the probe was surrounded by mylar sleeve158. The probe was positioned in front of the subject''s chest. FIG. 18shows the magnetocardiogram obtained from a human subject. It can beseen that the magnetocardiogram pattern compares favorably with theelectrocardiogram, of the same subject, shown in FIG.
 15. The symmetricSQUID 144 was magnetically coupled from one hole to flux transformer146, which is of the type shown in FIG.
 19. One advantage of themagnetocardiograph is that terminals need not be connected to thesubject. The symmetric SQUID can also be used as a detector of R.F.energy. In this application the device can be used to terminate asection of waveguide. When R.F. is applied to the waveguide its presenceis detected as a change in impedance of the tank circuit used with thedevice. Alternatively, the R.F. signal to be detected can be applied toone end of the device while a local oscillator signal of slightlydifferent frequency is applied to the other end of the device. The tankcircuit can then be tuned to the difference between these twofrequencies, to provide an I.F. pick off. I claim:
 66. The loosecoupling provides a high impedance connection to the tank circuit whichassures that the bias current is supplied from a constant current source(i.e. so that the bias level will not be a function of tank circuitimpedance). Audio oscillator 74 is connected by switch 76 to eitheroscillator 62 (position a) or choke 64 (position b). When the V-Icharacteristic (FIG. 4) is to be measured, switch 76 is thrown toposition a, and the audio signal is used to amplitude modulate theoutput of oscillator
 62. This, in effect, varies the bias currentsupplied to the SQUID. Audio oscillator 74 is also connected across theX (or horizontal) axis of XY scope
 78. Thus when switch 76 is connectedin position a, the variation in the amplitude of the SQUID bias currentis displayed on the X-axis on the face of scope 78 and the variation involtage across the tank circuit is displayed across the Y-axis (as shownin FIG. 4). When the field characteristic (FIG. 5) is to be measured,switch 76 is thrown to position b, and the audio signal, which isapplied via choke 64 and coaxial cable 86 to coil 82, is used to varythe field strength in the SQUID. In this manner, the variation in fieldstrength is displayed on the X-axis on the face of scope 78 and thevariation in voltage across the tank circuit is displayed across theY-axis (as shown in FIG. 5). USE OF THE SYMMETRIC SQUID AS AMAGNETOMETER AND GRADIOMETER By replacing the X-Y scope, in the circuitof FIG. 9, by a phase-locked detector and feeding the D.C. output of thephase-locked detector back to the SQUID, the system can be made tofunction as a lock-on magnetometer. The magnetometer circuit is shown inFIG.
 10. Components similar to those of the FIG. 9 circuit are similarlynumbered. A suitable phase locked detector 88 is a Princeton AppliedResearch Model HR-8, having the functional characteristics shown inTable
 2. TABLE 2 SENS. 50 volts FREQ. 95 Hz PHASE about 0 SIG Q about 10MODE INT. TIME CONST. 3 SEC REF ATTN. about 2 METER MONITOR - OUT ZEROOFFSET - 0 The phase locked detector output is coupled back to the tankcircuit coil 82 of the SQUID by coupling resistors 90 and 92, choke 64,and coaxial cable
 86. This output is also applied to chart recorder 98by the low pass filtering network comprising resistor 94 and capacitor96. In operation, changes in external field which are sensed by theflux-transformer will cause the phase-locked detector to produce a D.C.output signal which is fed back to coil 82 to produce a field in theSQUID which tends to oppose the external field until the output of thephase detector is nulled. The change in field strength needed to opposethe external field is recorded on chart recorder 98 as the magnetometeroutput. The external field is applied to the symmetric SQUID 80a byinserting in the other hole of the SQUID a coil 100 which is coupled toflux transformer
 102. The flux transformer-SQUID interconnection isrepresented schematically in FIG.
 11. The flux transformer is a closedloop comprising a superconductive loop, represented schematically bycoil 104, connected to coil
 106. Coil 106 is inserted into one of theholes of symmetrical squid 108 thereby magnetically coupling the fluxtransformer in the SQUID by mutual inductance M. A gradient-sensitiveflux transformer is shown schematically in FIG.
 12. The external fluxsensing portion of the gradient sensitive flux transformer isrepresented by two coils 110 and 112, of equal inductance, connected inseries opposition. Thus, there is no response to a uniform appliedfield, but a current proportional to the gradient of the field will beinduced. The gradient-sensitive flux transformer can be repreSentedspatially as shown in FIG. 13, where the two external superconductiveloops, 114 and 116, are aligned coaxially. The flux transformer respondsto delta Bz/ delta Z, where z represents distance along the line throughthe center of the two loops, and Bz represents the flux density of theexternal field as a function of z. The sensitivity of this device variesinversely with the distance between the sensing loops, l. Thegradient-sensitive flux transformer used in the preferred embodiment ofthe invention is shown in FIGS. 14 and 14A. The flux sensingsuperconductive loops, 120 and 124, are each formed by a single turn ofthin niobium sheet wrapped on quartz tube
 118. Connection to the SQUIDis made by twisted niobium leads
 122. These leads are connected to aflux sensing coil (not shown) which is inserted into one hole of thesymmetric SQUID. The flux sensing coil comprises five turns of No. 36(0.25 mm) niobium wire. The dimensions of the flux transformer of FIG.14 are shown in Table
 3. TABLE 3 DEVICE DIMENSION SIZE millimeters f 50g 50 h 50 i 50 Field measurements were made with the apparatus shown inFIG.